This invention pertains to multiple layer flexible sheet structures and the use of those sheet structures in flexible tubes of the type commonly used for packaging paste type products. Thick metal foil as a single layer has long been used to hold and dispense paste type products. However, metal tubes have definite shortcomings. Compared to other tube structures, metal tubes are very expensive, they dent easily and they tend to crack with a moderate amount of flexing.
More recently, a large share of the tube market has been taken by flexible sheet materials having a multiplicity of polymeric layers. Typical tubes have an inner heat sealable layer, an outer heat sealable layer, and a barrier layer interposed therebetween. Additional layers may further be used in conventional structures to provide other properties or qualities.
Layers of a non-polymeric nature, such as paper and thin metal foils may also be included in these sheet materials to provide specialized performance functions.
It is known, for example, to provide a layer of thin aluminum foil as a high quality barrier layer. When foil is used, it is common practice to use a highly adhesive polymer to adhere it to its adjacent layers.
It is also known to provide a layer of paper, which may provide dimensional stability, and also provide an aesthetically pleasing and aseptically clean appearing white background.
While known structures have experienced success in the commercial market place, there has been a certain degree of failure with existing tube structures; and certain products are so hard to hold that no polymeric tube structure has been acceptable for packaging them.
A particular problem of tube failure is attributed to failure of the tubes due to rough handling during shipping, wherein the tube sidewall splits, allowing the contents to ooze out.
Another problem is associated with chemical activity of product held in tubes having a layer of aluminum foil. The polymer between the foil and the product may develop small cracks known as stress cracks, due to chemical attack by the contained product. Stress cracks then allow the product to penetrate the polymer and either attack the foil, causing loss of adhesion between the foil and the adjoining layers; or the product may attack the interfacial bond between polymer layers thus causing loss of adhesion between these polymer layers. In either case, failure of the bond results in premature failure of the tube.
In addressing the stress crack problem, it is known to use a thick layer of ethylene acrylic acid (EAA) as the product contacting surface. While EAA does have substantial stress crack resistance, it is relatively expensive, and it is highly desirable to find a cheaper replacement material. Further, small particles of EAA are rubbed off its surface by the processing equipment during tube processing. These particles, known as polydust, are sometimes picked up by tubes, and may even become entrapped in the tube. While materials used for contact with hygenic products are preferrably acceptable for contact with the product, it is highly desired to avoid inclusion of any type of contamination into the product.
Thus it is desirable to find a material which has good stress crack resistance and also does not generate polydust in the processing equipment.
The ability of a tube to withstand rough handling is related to its ability to withstand a drop test described hereinafter as a tube drop test, in which a tube filled with product is repeatedly dropped until the tube fails. All tubes shipped in commerce may be expected to be subjected to rough handling, essentially independent of the product contained and are thus subject to handling stress as exemplified in the tube drop test. Economic construction of tubes consistently capable of passing the drop test has remained a problem.
While numerous attempts have been made in the packaging industry to resolve these problems in tube construction, and while certain polymers and, in particular polyethylenes, have been offered to the packaging industry with suggestions of improved stress crack resistance, the problems of stress crack resistance and tube sidewall strength have remained unresolved. Thus it remained for the inventor to resolve these problems by designing improved sheet structures, and tubes made therefrom, after extensive testing of many alternative structures.